Demand for environmentally-friendly, clean liquid fuels with low sulfur and aromatic hydrocarbon contents has increased in recent years from the viewpoint of reducing environmental load. Fischer-Tropsch synthesis (hereinafter referred to as “FT synthesis”) using carbon monoxide and hydrogen as feedstock has been studied in the petroleum industry as a process for production of clean fuels. FT synthesis is a highly promising method as it allows production of liquid fuel stocks rich in paraffins and containing no sulfur.
Because of their high normal paraffin contents, however, fuel stocks obtained by FT synthesis cannot easily be used directly as fuels. More specifically, such fuel stocks have insufficient octane numbers for use as gasoline for automobiles, while their cold flow properties are insufficient for use as gas oils.
Hydrorefining techniques, whereby normal paraffins in fuel stocks are converted to isoparaffins, have been investigated with the goal of increasing the octane numbers and improving the cold flow properties of fuel stocks obtained by FT synthesis. Known catalysts for use in such hydrorefining techniques include catalysts comprising palladium supported on silica-alumina (see Patent document 1, for example) and catalysts comprising cobalt and molybdenum supported on silica-alumina (see Patent document 2, for example).    [Patent document 1] U.S. Pat. No. 5,378,348    [Patent document 2] International Patent Publication No. WO01/057160